For bromination of benzene

This is not unreasonable since it was found that the second-order rate coefficient for bromination of benzene by bromine acetate at 60 °C was 3.20 xlO-2, i.e. twenty times faster than under the above conditions for which 103k2 ranged from ... 

Two excellent reviews <71AHC(13)235, 72IJS(C)(7)6l) have dealt with quantitative aspects of electrophilic substitution on thiophenes. Electrophilic substitution in the thiophene ring appears to proceed in most cases by a mechanism similar to that for the homocyclic benzene substrates. The first step involves the formation of a cr-complex, which is rate determining in most reactions in a few cases the decomposition of this intermediate may be rate determining. Evidence for the similarity of mechanism in the thiophene and benzene series stems from detailed kinetic studies. Thus in protodetritiation of thiophene derivatives in aqueous sulfuric and perchloric acids, a linear correlation between log k and —Ho has been established the slopes are very close to those reported for hydrogen exchanges in benzene derivatives. Likewise, the kinetic profile of the reaction of thiophene derivatives with bromine in acetic acid in the dark is the same as for bromination of benzene derivatives. The activation enthalpies and entropies for bromination of thiophene and mesitylene are very similar. 

Exercise 22-9 Devise an experimental test to determine whether the following addition-elimination mechanism for bromination of benzene actually takes place. 

Exercise 22-14 Aluminum chloride is a much more powerful catalyst than ferric bromide for bromination of benzene. Would you expect the combination of aluminum chloride and bromine to give much chlorobenzene in reaction with benzene Explain. 

In contrast with chlorination, bromination of thiophene always gives substitution products exclusively, and no addition products have been isolated under a variety of experimental conditions.82 Lauer83 first studied the kinetics of the reaction of thiophene with molecular bromine however, the reported value for the rate relative to benzene (2 x 104) is not reliable, because of the uncorrected value of k for bromination of benzene (for a discussion of this point, see Marino72). Later, the rate of bromination of thiophene relative to benzene in acetic acid was determined as 1.9 x 109, by comparing the times necessary to achieve 10% reaction in the bromination of thiophene and mesitylene, under the same conditions.72... 

An electrophilic aromatic substitution reaction begins in a similar wav. but there are a number of differences. One difference is that aromatic ring.< are less reactive toward electrophiles than alkenes are. For example, Br in CH2CI2 solution reacts instantly with most alkenes but does not react at room temperature with benzene. For bromination of benzene to take plai a catalyst such as PeBrj is needed. The catalyst makes the Br2 molecu.. more electrophilic by polarizing it to give an FeBr4" Br species that reaci as if it were Br. ... 

The halogen carriers or aromatic halogenation catalysts are usually all electrophilic reagents (ferric and aluminium haUdes, etc.) and their function appears to be to increase the electrophilic activity of the halogen. Thus the mechanism for the bromination of benzene in the presence of iron can be repre-sfflited by the following scheme ... 

The heats of formation of Tt-complexes are small thus, — A//2soc for complexes of benzene and mesitylene with iodine in carbon tetrachloride are 5-5 and i2-o kj mol , respectively. Although substituent effects which increase the rates of electrophilic substitutions also increase the stabilities of the 7r-complexes, these effects are very much weaker in the latter circumstances than in the former the heats of formation just quoted should be compared with the relative rates of chlorination and bromination of benzene and mesitylene (i 3 o6 x 10 and i a-Sq x 10 , respectively, in acetic acid at 25 °C). 

Figure 16.3 An energy diagram for the electrophilic bromination of benzene. The overall process is exergonic.

Subsequently, rate coefficients were determined for the zinc chloride-catalysed bromination of benzene, toluene, i-propyl-benzene, r-butylbenzene, xylenes, p-di-f-butylbenzene, mesitylene, 1,2,4-trimethyl-, sym-triethyl-, sym-tri-f-butyl-, 1,2,3,5-and 1,2,4,5-tetramethyl- and pentamethylbenzenes, all at 25.4 °C and in acetic acid, and it was shown that the reaction was inhibited by HBr.ZnCl2 which accumulates during the bromination and was considered to cause the first step of the reaction (formation of ArHBr2) to reverse320. The second-order coefficients for bromination of o-xylene at 25.0 °C were shown to be inversely dependent upon the hydrogen bromide concentration and the reversal of equilibrium (155)... 

An investigation of the relative rates of bromination of benzene, toluene, m-and p-xylene by bromine in acetic acid, catalysed by mercuric acetate, revealed relative rates almost identical with those obtained with molecular bromine322, though as in the bromination of biphenyl by bromine acetate (p. 129) it is quite inconsistent for a much more reactive electrophile to have the same selectivity. Relative rates were (molecular bromination values in parenthesis) benzene 1.0 toluene, 480 (610) p-xylene, 2.1 x 103 (2.2 x 103) m-xylene 2.0 x 10s (2.1 x 10s). 

An important use of the dediazoniation reaction is to remove an amino group after it has been used to direct one or more other groups to ortho and para positions. For example, the compound 1,3,5-tribromobenzene cannot be prepared by direct bromination of benzene because the bromo group is ortho-para directing however, this compound is easily prepared by the following sequence ... 

Enough mutual polarisation can apparently result, in (8), for (9) to form, but polarisation of the bromine molecule may be greatly increased by the addition of Lewis acids, e.g. AlBr3 (cf. bromination of benzene, p. 138), with consequent rise in the rate of reaction. Formation of (9) usually appears to be the rate-limiting step of the reaction. 

Lewis acids such as FeCl3 and ZnCl2 are also useful catalysts. For example, bromination of benzene by Br2 in the presence of FeBr3 can be shown as... 

Exercise 22-15 a. The bromination of benzene is catalyzed by small amounts of iodine. Devise a possible explanation for this catalytic effect. 

Because the rate of substitution varies with position, in a benzene derivative it is more informative and frequently more useful to talk about partial rate factors than about relative rates. A partial rate factor is defined as the rate at one particular position in the benzene derivative relative to the rate of substitution at one position in benzene. Let us, for example, calculate the para and meta partial rate factors (pf and mf, respectively) for bromination of toluene with bromine in aqueous acetic acid. Toluene brominates 605 times faster than benzene under these conditions. The product is 66.8 percent p-, 0.3 percent m-, and 32.9 percent o-bromotoluene. Attack at the para position of toluene occurs 0.668 x 605 times as fast as attack at all six positions of benzene but (0.668 x 605 x 6 = 2420) times as fast as at one position of benzene. Therefore pfCH for bromination of toluene under these conditions is 2420. There are only three times as many total carbons in benzene as meta carbons in toluene. Therefore mfca3 = 0.003 x 605 x 3 = 5.5. The definitions of the partial rate factors for monosubstituted benzenes (—R) are given in Equations 7.78-7.80. 

An electrophile — an electron-seeking reagent — is generated. For the bromination of benzene reaction, the electrophile is the Br+ ion generated by the reaction of the bromine molecule with ferric bromide, a Lewis acid. 

There are principally two different approaches of correlating experimental rate data of electrophilic substitution with reactivity indices (1) Correlating the index with the rate data of a given reaction, e.g. bromination. For example, a satisfying correlation of Dewar reactivity numbers with the log of rate constants of the bromination of benzene, naphthalene (1- and 2-position), biphenyl (4-position), phenanthrene (9-position), and anthracene (9-position) has been observed [55]. In correlations of this type the reactivity index corresponds to the reactivity constant in the Hammett equation while the slope of the linear correlation corresponds to the reaction constant (see also Sect. 3) (2) correlating the index with experimental a values. 

Aromatic rings undergo electrophilic substitution, for example the bromination of benzene (Following fig). The reaction involves an electrophile (Br+) replacing another electrophile (H+) with the aromatic ring remaining intact. Therefore, one electrophile replaces another and the reaction is called an electrophilic substitution. 

There is neither a partial positive nor a partial negative charge on the two nonequivalent positions 1 and 2 of naphthalene, which are poised for electrophilic substitution. One might consequently predict that electrophiles react with naphthalene without regiocontrol. Furthermore, this should occur with the same reaction rate with which benzene reacts. Both predictions contradict the experimental results For example, naphthalene is brominated with a 99 1 selectivity in the 1-position in comparison to the 2-position. The bromination at Cl takes place 12,000 times faster and the bromination at C2 120 times faster than the bromination of benzene. 

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